Low-carbon steels having a yield strength of approximately 170 megapascals (MPa) and excellent deep drawing ability are used in a variety of industries, e.g. the automobile industry. However, despite their forming and cost advantages over high-strength steels, the relatively low-strength level results in the crash performance of such materials being mainly dependent on a thickness of a sheet thereof. As such, 1st generation advanced high-strength steels (AHSS) have been developed in order to reduce the weight of automotive components and thus afford for improved vehicle fuel efficiency.
Among the 1st generation AHSS, dual phase steels are increasingly being used in the vehicle components for “lightweighting” of automobiles. The excellent strength-ductility balance gives a large formability range for comparable high tensile strength HSLA steels and thus make them one of the most attractive choices for automobile weight reduction. Further optimization of material designs requires automobile manufacturers opting for AHSS grades from the higher end of the spectra in terms of tensile strength where dual phase steels are an able choice for incorporation in the current assemblies. In particular dual phase steels can be produced by subjecting low-carbon steels to an intercritical anneal followed by sufficiently rapid cooling. It is appreciated that an intercritical anneal refers to annealing the steel at a temperature or temperature range below the materials Ac3 temperature and above the Ac1 temperature where the microstructure consists of ferrite and austenite, thereby affording for the rapid cooling to transform the austenite into martensite such that a predominantly dual phase ferrite-martensite microstructure is produced.
It is known in the art that alloying elements such as manganese, chromium, molybdenum, and vanadium can be used to reduce the rate of cooling required for the transformation of the austenite to martensite. For example, Mo has been an effective alloying element, especially for coated sheets, for imparting quench hardenability. Molybdenum additions also have the added benefit of not being prone to selective oxidation during annealing—as compared to Cr, Mn, and Si—and thus not hampering surface characteristics of coated dual phase steels. The added alloying elements circumvent the requirement of high cooling rates on a production line to obtain martensite as a low temperature transformation product in a ferritic matrix. The alloying elements become more consequential in the case of dual phase steels having tensile strength above 980 MPa which require high volume fractions of the hard phase martensite. However, the addition of such alloying elements, with Mo being most expensive, naturally increases the cost of the steel.
Three basic methods are known for the commercial production of dual phase steels. First, an as-hot-rolled method produces the dual phase microstructure during conventional hot-rolling through the control of chemistry and processing conditions. Second, a continuous annealing approach typically takes coiled hot or cold rolled steel strip, uncoils and anneals the steel strip in an intercritical temperature range in order to produce a ferrite plus austenite microstructure/matrix. Thereafter, sufficiently rapid cooling higher than the critical cooling rate for the steel chemistry is applied to the strip to produce the ferrite-martensite microstructure. Finally, the third method batch anneals hot or cold rolled material in the coiled condition.
The temperature or temperature range of the intercritical anneal is important since for a given alloy composition the intercritical anneal temperature controls or determines the amount of austenite, and its carbon content, that can be transformed to martensite.